Subscriber Loop Design

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The cables that connect the telephone handsets or other devices to the local switching office or end office is referred as subscriber loop or local loop. Every suscriber has his own pair of wires to the local switching office. Twisted pair local loop is an excellent transmission medium for analog voice signals. But it is limited to low frequency audio signals. The introduction of fiber cable needs a device at subscriber premises to convert electrical energy into light energy and this is the additional cost to the customer. But for high speed data transmission, switched cable TV, videophone, teleconferencing the fiber optic local loop has become essential.

One end of each subscriber loop is terminated on a Main Distribution Frame (MDF) at the exchange. The drop wires (DW) from the telephones are connected to the distribution point (DP) which is located near the subscriber's premises. The distribution points at various locations are connected together by a distribution cables (DC) and terminated to the feeder points (FD). The DC carries 10-500 pair of wires. Many feeder points related to a particular geographical area connected by a branch feeder (BF). From BF, through main feeder, all the subscriber loops are connected to MDF at the end office or local exchange. Typically, the MF carries 1002000 pair of wires to MDF. For the purpose of flexible interconnection such as transfer from location to other location or within the geographical area, the subscriber pair and exchange pairs are interconnected at the MDF by means of jumpers.

3.5.1. Fundamental Characteristics

The subscriber loop is the most common interface in the network. The fundamental characteristic of this interface are.

Battery. To enable dc signalling and to provide bias current for carbon microphone, a battery of about 48 V is connected to subscriber loop at exchange.

Overvoltage protection. Protection of equipment and personal from lightning strikes and power line induction or shots.

Ringing. Application of a 20 Hz signal at 86 V rms for ringer excitation.

Supervision. Supervise the network by detecting the off hook/on hook and flow/no-flow dc current.

Coding. In the case of digital end office, analog to digital coding and digital to analog decoding functions necessary.

Hybrid. For two wire to four wire conversion, hybrid in necessary. Test. Line test toward the subscriber disconnection of the switch.

The first letter of the above characteristics are coined together which is commonly known as BORSCHT.

3.5.2. Limiting Factors of Subscriber Loop Design

There are two limiting factors we have to consider while designing a subscriber loop. First one is the attenuation. The attenuation refers to the energy loss in the line at a reference frequency, measured in decibels. The reference frequency is 1000 Hz in America and 800 Hz in Europe. If the length of the loop increases the attenuation also increases. The attenuation limit of the subscriber loop is normally 6 dB.

The second limiting factor is voltage drop. If the battery voltage is kept constant with increase in length, the effectiveness of the signalling and conversation will be limited. This is due to IR drop of the line. The IR drop of the line varies with resistances of the battery used in the system, telephone set resistance and the allowable resistance of the subscriber loop.

The maximum allowable resistance in the subscriber loop and the loop resistance limit is calculated as follows. The loop resistance limit is calculated as follows. The loop resistance limit is used to determine the cable length and the cable gauge required.

where Rm = maximum allowable resistance of subscriber loop.

VB = Battery voltage

Ic = minimum current required for proper operation of carbon microphone

The loop resistance limit is RL = Rm - (RB + RT) ...(3.11)

where RB = approximate resistance maintained at the battery protect against short circuit in the wire between subscriber and local office

Rt = Telephone set resistance. Example 3.1. If the minimum current required for carbon microphone is 23 mA, battery voltage is 50 V, the battery resistance is 400 ohm and the telephone set resistance is 200 ohms, calculate the loop resistance limit.

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